1. Field of the Invention
The present invention relates generally to photonic links, and in particular to photonic RF links.
2. Discussion of the Related Art
Photonic RF links have been demonstrated for antenna remoting, RF distribution and CATV broadcasting. An externally modulated photonic RF link, consisting of a high power laser source, a modulator and a photodetector, has superior performance over a directly modulated link. In an externally modulated photonic link, the link gain and spur free dynamic range (SFDR) have been largely limited by the modulators. Low modulation voltage, low optical insertion loss, high power handling capability, and linearized transfer function are all of critical importance to the link performance. The bandwidth of the modulator also has to match to the desired link bandwidth, leading to many difficult design trade-offs. Conventional modulator technology is not able to simultaneously meet all of the desired performance specifications.
For RF power distribution or remoting, a photonic RF source is a viable substitute for conventional externally modulated photonic RF links. In order to obtain high quality RF signals through photonic generation, a phase locked loop (PLL) is commonly introduced to control the random phase drift between the lasers. In these systems, the generated RF signal can be used to compare to a reference signal from a synthesizer, and the phase error voltage will be used to adjust the laser frequency, which functions as a photonic voltage controlled oscillator (VCO). For extended frequency tuning range, external RF mixers can be introduced to down convert the generated RF signal.
A photonic RF source can be tuned over a very broad bandwidth by virtue of the large frequency difference between the optical wave and the RF wave, subject to the availability of high speed photodetectors. The PLL, however, typically presents the limiting factor for broadband tunability, since external RF (microwave) mixers are limited at around 40 GHz. Commercial phase detectors are limited with a bandwidth lower than 1 GHz due to the response limit of current CMOS technology. Thus, there exists a need for a photonic RF generation system with improved bandwidth capabilities.